Gregor Mendel conducted experiments with pea plants to study inheritance of traits. He found that traits are inherited in distinct units (now called genes) and can exist in different forms (alleles). Through his experiments, he developed two laws of inheritance: the Law of Segregation, which states that organisms pass only one allele to offspring for each gene, and the Law of Independent Assortment, which explains that genes assort independently during gamete formation. Mendel's work established foundations of classical genetics and heredity of traits.

1. Mendelian Genetics
How Are Traits Inherited?

2. Gregor Mendel
• Raised on farm and understood the value of
plant breeding.
• At 21, entered priesthood and studied plant
breeding in a monastery in the Czech Republic.
• Loved to read especially about natural sciences
and was aware of Darwin’s findings.
• Studied the inheritance of traits in pea plants.
• Considered the FATHER OF GENETICS!!!!!

3. Gregor Mendel
• Studied traits that occur
in distinct forms.
• Developed true-breeding
varieties
– When bred amongst
themselves these plants
produced offspring identical
to the parent for that trait.
• Used mathematical
analysis in his studies.

4. Definitions
• Gametes: reproductive
cells produced by
sexually reproducing
organisms.
– Two types:
• male gametes = sperm
– In plants: contained in
pollen
• Female gametes = eggs
– In plants: contained in
ovules
– Ovules contained in
carpels

5. Fertilization
• Fertilization: fusion of
egg and sperm
– Self-fertilized: fusion of
sperm and egg from same
plant
– Cross fertilized: fusion of
egg and sperm from two
different plants
• Produced hybrids
• F1: first generation
• F2: second generation

7. Mendel’s Interpretations
• Modified to incorporate today’s vocabulary
– Genes: the hereditary information that determines a
single trait
– Alleles: alternate forms of a gene
• When an organism inherits two identical alleles
for a trait, organism is said to be homozygous for
the trait.
• When an organism inherits to different alleles for
one trait, the organism is called heterozygous for
the trait.

8. Why were all the seeds in the F1
generation round?
• Dominant vs. Recessive Allele
– Dominant: an allele that is expressed
whenever it is present
– Recessive: an allele that is masked
whenever the dominant allele is present.
• Dominant and recessive alleles influence
an organism’s phenotype

9. Genotype and Phenotype
• Genotype: Genetic makeup of an individual. It
is determined by the alleles present for each
trait.
• Phenotype: Physical appearance of a trait. It is
the expression of the genotype.

10. Can you look at someone and
determine their genotype?
• Testcross is used to determine the
genotype of an individual
– Testcross: crossing an organism with
unknown genotype with one that is
homozygous recessive for the trait.
– Example:
• Plant that has round (R) seeds crossed with one
that is homozygous recessive (rr) for wrinkled
seeds
R? x rr

11. Using Punnett Squares to Predict
the Inheritance of Sickle Cell
Anemia

12. Sickle Cell Anemia
• Due to presence of recessive allele
denoted by HbS.
• Causes distortion of red blood cells into
long, thin sickles.
• Causes pain, tissue damage, and cells to
rupture prematurely.
• Carriers: have heterozygous genotypes
and do not manifest symptoms.

13. Punnett Squares
• Used to predict
possible offspring
genotypes
• Place alleles for each
parent on each side
HbA/ HbA x HbS/ HbS HbA
HbA
HbS HbS
Mother’s genotype Father’s genotype

14. Predict inheritance
• Given parents’
genotypes, you can
predict offspring’s
genotypes and
phenotypes
• HbS/ HbS = homozygous
recessive results in sickle
cell anemia
• Both HbA/ HbS
(heterozygous) and HbA/
HbA (homozygous
dominant) are normal.
What are the genotypic and phenotypic ratios for this cross?

16. Mendel’s Laws
• Based on Mendel’s data, he formulated
two laws:
– Law of Segregation: A parent contributes only
one of its alleles for a trait to each offspring.
– If parent is heterozygous for a trait, the
particular allele donated to the offspring is
random.

17. Mendel’s Laws
• Law of Independent
Assortment: Alleles
of one gene are
passed to offspring
independently of the
alleles of other genes.
– Applies to the
inheritance of two or
more genes
simultaneously.

18. • Can use the inheritance of traits in fruit flies to
illustrate the law of independent assortment.
• Dihybrid cross
– A fertilization in which the parents differ in two distinct
traits or characteristics.

21. Probability
• Can be used to better
understand
segregation and
independent
assortment.
• Look at coins first…

22. Why Aren’t Members of the Same
Species Identical?
• Sources of Genetic Variation
– Law of Independent Assortment
– Mutation

23. Law of Independent Assortment
• Allows for new gene combinations or
genetic recombination
• Can mathematically predict the possible
combinations
– Number of possible genotypes = 2n
where n = the number of genes or traits
considered
– Example: considering 100 traits:
• 2100 = 1.26765 x 1030

24. Do Mendel’s Laws Always Apply?

25. Incomplete Dominance
• A condition where all
three genotypes are
expressed.
• Phenotypic level:
contradicts Mendel’s
conclusions.
• Genotypic level:
consistent with
Mendel’s laws.

26. Lethality
• A condition in which
the inheritance of a
lethal combination of
alleles results in
death of the
organism.
• Lucien Cuenot
studied inheritance of
coat color in mice.
• Huntington’s disease.

27. Pleiotropy
• A single gene affects
two or more traits.
• Examples:
– One gene affects
whether seed coat is
round/wrinkled
– Cat fur

28. Polygenic vs. Monogenic
inheritance
• Polygenic: A trait
affected by many
genes.
– Examples: Height,
weight, skin color
• Monogenic: Traits
determined by single
gene with two alleles.
– Examples: Flower
color in four-o’clock
plants